A magnetic sensor using a magneto-resistive effect element can detect, in a non-contact manner, displacement of a detection object provided with a magnetic field generation mechanism, and is used as a magnetic encoder and a magnetic rotation-angle detection sensor. Among magneto-resistive effect elements, a magneto-resistive effect element, using a giant magneto-resistive effect (hereinafter described as GMR) film and referred to as Spin-Valve (hereinafter described as SV) type, is useful for the detection of rotation angle.
As described in Patent Literature 1, an SV type GMR film has a basic structure formed of an antiferromagnetic layer/a pinned layer/a nonmagnetic layer/free layer. The pinned layer is exchange-coupled to the antiferromagnetic layer formed adjacently to the pinned layer, so that the magnetization direction of the pinned layer is fixed to one direction. On the other hand, the magnetization direction of the free layer is changed according to an external magnetic field.
Therefore, when a rotating magnetic field signal having a comparatively small intensity is applied to the SV type GMR film, the relative angle between the magnetization of the pinned layer and the magnetization of the free layer is changed, so that an electric output corresponding to the change of the relative angle can be obtained. That is, a function as a rotation angle sensor can be obtained in such a manner that a magnetic field generation mechanism, such as a permanent magnet, is mounted to a rotation detection object, and that the rotating magnetic field generated in synchronization with the rotational movement of the rotation detection object is converted into an electric signal.
In particular, when a magneto-resistive effect element is used to detect an absolute angle of the rotation detection object, it is important that the detection sensitivity of the magneto-resistive effect element is isotropic with respect to the rotating magnetic field, and that the magneto-resistive effect element has a small detection error in any direction of the magnetic field. Further, it is also necessary that the detection angle of the magneto-resistive effect element is not deviated due to a change in the operating environment temperature.
In order to fulfill such requirements, there is proposed a rotation angle sensor which includes a plurality of magneto-resistive effect elements, each using a SV type GMR film having a pinned layer magnetized in a different direction, and in which the plurality of magneto-resistive effect elements are connected in the form of a bridge circuit.
Generally, it is difficult that only specific portions of a pinned layer of an SV type GMR film are respectively magnetized in a plurality of desired directions. Therefore, in order to obtain magneto-resistive effect elements having a plurality of magnetization directions, it is necessary to produce beforehand a plurality of magneto-resistive effect elements, each using an SV type GMR film having a pinned layer magnetized in a different direction. Then, the magneto-resistive effect elements are cut into separate element units, and the separate element units are mounted in a bridge circuit.
On the other hand, it is also strongly required that the rotation angle sensor is stably operated in a high temperature environment. From the viewpoint of maintaining the thermal stability of the SV type GMR film, how the magnetization of the pinned layer is strongly fixed becomes a bottleneck. The exchange coupling between the pinned layer and the antiferromagnetic layer generally disappears at a temperature of about 250° C. to 320° C., and hence it has been difficult to realize sufficient thermal stability of magnetization of the pinned layer.
In order to solve the above-described problems, Patent Literature 2 discloses, as another method for fixing magnetization of the pinned layer, a structure of a pinned layer which does not include an antiferromagnetic layer and which is formed of a first ferromagnetic layer/an antiferromagnetic coupling layer/a second ferromagnetic layer. For example, when a laminated structure composed of Co/Ru/Co is formed by a suitable production method so as to have a suitable thickness, the two Co layers are strongly interlayer-coupled to each other in an antiferromagnetic manner via the Ru layer. As a result, the magnetizations of the two Co layers, which are arranged antiparallel to each other, are hardly changed by an external magnetic field. This is applied in the technique described in Patent Literature 2. Here, such pinned layer structure is referred to as a self-pinned type.
In a self-pinned type pinned layer, the magnetization can be stably fixed at a higher temperature as compared with an ordinary pinned layer using an exchange coupling with an antiferromagnetic layer. Therefore, the self-pinned type pinned layer has a structure which is preferred to solve the above-described problems.
Further, the self-pinned type pinned layer has a significant advantage also in relation to a method for defining the magnetization direction.
Generally, the magnetization direction of an ordinary pinned layer using an exchange coupling with an antiferromagnetic layer is defined by annealing in which is performed under application of a magnetic field after a GMR film is formed. That is, in this method, it is difficult to magnetize a pinned layer on a same substrate in respective different directions.
On the other hand, the self-pinned type pinned layer can be magnetized in any direction by changing the direction of the magnetic field applied at the time of film formation. For this reason, a plurality of GMR films, each of which has a pinned layer magnetized in a different direction, can be formed in a same substrate.
Therefore, a step of forming magneto-resistive effect elements by using micro processing, and then a step of connecting electrode terminals to a bridge circuit formed by using the magneto-resistive effect elements can be performed in a same substrate. Therefore, a magnetic sensor can be produced by a simple production flow.
Patent Literature 3 describes a magnetic sensor produced by using the above-described method.
In Patent Literature 4, a method using local heating is described as a technique to magnetize a self-pinned type pinned layer in a plurality of directions.
In Patent Literature 5, a method is disclosed, which relates to control of the magnetization direction of a pinned layer, and which induces a uniaxial magnetic anisotropy by using texture formation by etching.